In a binary adder, two binary inputs are generally introduced to each stage in the adder. A carry may also be introduced to each such stage from the preceding stage in the adder. The two (2) binary inputs and the carry are then added in such stage to determine whether a carry will be introduced to the next stage in the adder.
Considerable work has been performed in the prior art to develop and perfect a satisfactory binary adder. In one type of binary adder in the prior art, each successive stage in the adder operates in sequence. When a stage operates, it adds the binary inputs to that stage and the carry to that stage from the preceding stage, producing a sum and a carry. The next stage adds that carry and the binary inputs to that stage to provide a sum and a carry to the following stage in the adder. Each of the stages accordingly operates sequentially after the operation of the previous stage. As will be seen, this type of binary adder is disadvantageous because it is slow. However, this type of binary adder has the advantage that it is occupies a minimal amount of space on an integrated circuit chip.
In another type of binary adder of the prior art, electrical circuitry is included in the adder for anticipating the carry from each stage to the next. Because of such anticipation, this type of binary adder is relatively fast in its operation. However, in order to provide such anticipation, the circuitry in the binary adder is quite complex because each stage must have logic that looks ahead in time to the inputs of all proceeding stages. Furthermore, the circuitry occupies a relatively large area on an integrated circuit chip.
It has been known for some time that it would be desirable to provide circuitry which provides a binary carry between successive stages in a binary adder and which is fast, simple and compact. On the basis of such knowledge, a considerable effort has been made for a considerable number of years to provide carry circuitry which meets such criteria. Such efforts have, as a practical matter, been fruitless until now.